After going through this article, you'll be able to download any video from youtube or google-video without using any online site or software tool.
But before that you've to understand the concept behind showing video on internet browser.
Actually, whenever you open a web-page having video, a flash video player in the form of swf is downloaded in your system's cache. When you click on the button or link to play the video, the player starts downloading a .flv file from the site to your system's cache.
Note the flash video player can play only .flv files and is programmed to start playing video if even a bit of the whole video-file is downloaded. This technique is known as video streaming.
Your system capture this video stream in the cache, from where the flash video player plays it.
In Internet Explorer cache it is displayed as a filename 'get_video' with a suffix of very very long encrypted code.
But you don't have to worry about that.
Just open your internet cache and search for the file name having a string 'get_video'. If you have watched the whole video on flash video player then only you'll find the required full video file in the cache. Otherwise you'll get a file having video upto where it is downloaded.
When you'll find the required file then copy that file somewhere else as you can't play that file right there because Window doesn't give permission to do that.
After copying that file somewhere else, rename the file to something meaningful & add the extension .flv.
Now you need an flv player to play this file. For this purpose, VLC player would be the best.
You can get VLC player from this site, http://www.videolan.org/vlc/
So download & install VLC player at your system and play your .flv file whenever you want.
There is a problem you could face, if you have watched many videos. As in that case there will be many get_video file present in the cache. So to get rid of the headache of finding the required one either clear the cache before opening the selected video on web-page or copy all the get_video file that is present in the cache to somewhere else and watch one by one & search for the required one.
If you don't want to watch the video but want to download the video then you can do that by using Opera internet browser. As it provides some advanced features.
You can download Opera from this site, http://www.opera.com/download/
After downloading & installing the Opera browser, open it and type the required url and open the site. Click on play button. Open another tab. Then go to Tools>Advanced>Cache & click on it.
Now find "get_video" string via Ctrl+F. You'll get the link to that video file. Click on that link & after few minutes a download window will pop up. Download this file on your desktop and rename it to something meaningful with an extension .flv.
You can play this file with VLC player.